In the packaging of food products in thermoplastic film there have been attempts to address the problem of puncture of the film in various ways to assure both isolation from the atmosphere and to prevent leakage of liquids which may be present in the package. For meat products which contain bones which can be placed in a fixed relationship to the packaging, the practice has developed of applying a “patch” of polymer film on top of the primary packaging. Typically this “patch” has been adhered to the primary packaging film by a corona treatment of the facing surfaces or by covering the facing surfaces with an appropriate adhesive. Over time the size of the “patch” has evolved to cover almost the entire surface of the package thus making the location of bone less critical. However, this approach is not suitable for use with the semi-automated packaging approach wherein a cavity is thermoformed into a thermoplastic web, the food product is placed into the cavity and a portion of a second thermoplastic web is heat sealed over the open mouth of the cavity such as is taught by U.S. Pat. No. 3,956,867 to U Z et al. of Multivac. Another approach has been to simply drop “patches” into the already formed cavity before the food product is placed in the cavity in order to provide extra puncture resistance.
The approach of simply increasing the thickness of the web used to form the cavity has not met with much success. This approach is limited by the need to still be able to thermoform the web to form the cavity and the need to limit the expense of the packaging material. The need to control the expense also limits the selection of more puncture resistant materials.
Reasonable puncture resistance can be obtained using oriented shrink film which has been partially annealed to render it compatible with hot forming. But the present invention provides a film structure which does not require the use of such films.